Urethane coating compositions using a polyisocyanate as a curing agent are excellent in chemical resistance and flexibility of the resulting coating film. Particularly, when polyisocyanates obtained from aliphatic or alicyclic diisocyanates are used as the curing agent, the coating film is excellent also in weathering resistance. Therefore, these polyisocyanates are used in cold- or heat-curing two-pack urethane coating compositions or heat-curing one-component urethane coating compositions in a wide variety of the fields such as constructions, heavy-duty anticorrosion, automobiles, industries and repairing.
Particularly, polyisocyanates obtained from hexamethylene diisocyanate (hereinafter referred to as “HDI”) which is an aliphatic diisocyanate and isophorone diisocyanate (hereinafter referred to as “IPDI”) which is an alicyclic diisocyanate are used widely because they are industrially easily available and the coating films obtained using them are excellent in weathering resistance, and HDI and IPDI have characteristics different from each other. Since HDI has two primary isocyanate groups, it is high in reactivity, and since HDI has six methylene groups between the isocyanate groups, the coating film obtained from coating compositions containing it is excellent in flexibility, while it is sometimes difficult to attain high hardness of the coating film. On the other hand, IPDI has two isocyanate groups which are primary and secondary isocyanate groups, and it is sometimes difficult to attain high reactivity, but since it has a cyclic structure between the isocyanate groups, the coating compositions containing IPDI are excellent in drying property and the resulting coating film is high in hardness.
There are earnestly desired those polyisocyanates which have both the advantages of reactivity of HDI and drying property of coating compositions containing IPDI. Some proposals have been made thereon. Patent Document 1 and Patent Document 2 disclose polyisocyanate compositions comprising a mixture of HDI prepolymer and IPDI prepolymer. According to the technologies disclosed, it is difficult to overcome the low reactivity of the IPDI prepolymer because the respective isocyanate prepolymers are present independently in the composition.
Biuret bond-containing polyisocyanates obtained from HDI and IPDI are disclosed in Patent Document 3, isocyanurate bond-containing polyisocyanates obtained from HDI and IPDI are disclosed in Patent Document 4, and isocyanurate bond-containing polyisocyanates obtained from HDI and IPDI modified with a monoalcohol are disclosed in Patent Document 5. These are all technologies relating to polyisocyanates in which HDI and IPDI are both incorporated into the molecules, and consequently the number of primary isocyanate groups having high reactivity per one molecule of the polyisocyanates decreases, and sometimes the reactivity (curability) is deteriorated.
On the other hand, Patent Document 6 discloses a polyisocyanate which is large in average number of isocyanate group (the statistical average number of isocyanate group in one molecule) and which is derived from HDI and a polyol. The crosslinkability of the polyisocyanate has been enhanced, but high drying property has been further required.
Patent Document 1: U.S. Pat. No. 5,137,972
Patent Document 2: JP-A-2002-293873
Patent Document 3: JP-B-4-71908
Patent Document 4: U.S. Pat. No. 4,419,513
Patent Document 5: U.S. Pat. No. 5,258,482
Patent Document 6: U.S. Pat. No. 5,817,732